Grab a little container of some sort and go on a treasure hunt!
Collect any promising bits and pieces you come across (that you are willing to dedicate to craft-only use). A few good places to start are your kitchen (or office) junk drawer, that little box or jar in the craft room where you put stuff that has no real use (but that you can't bear to throw away), and the jar where you store rancom bits of hardware (or the bottom of your tool box, or wherever else such odds and ends accumulate).
Look for (but don't limit yourself to) needles, buttons, screws, nuts, bolts, beads, bead caps, scraps from nonfunctional mechanical objects, gears, paper clips, grommets, washers, pieces of pens, scraps and findings from broken (or out-dated) fashion jewelry, chains, orphan earrings, wires, springs, coins, seashells, and small stones, pebbles, or gravel.
Oven-safe bits and pieces are best. (Think metal and glass.) However, even if something's not oven-safe, you may still be able to use it, so if you think you'll love the texture it makes, pick it up. (See "Additional Ideas", below, for more information on how to give these pieces polymer clay handles.)